Renewable energy, which is drawing attention as future alternative energy, requires an energy storage system (ESS) which allows for efficient electric energy storage in and supply to a power grid in order to improve electric power quality and efficiency.
As an energy storage device for such an ESS, a lithium battery, for example, a lithium ion secondary battery, is drawing attention. However, a lithium ion secondary battery is limited as an energy storage device for a high capacity electric power storage system due to uneven distribution of lithium resources, a high price, and stability problems such as the danger of explosion by exposure to air.
The unit price per unit energy density of a lead-acid battery, which is used as an alternative, is about 25% of that of a lithium ion secondary battery. However, as the lifecycle is deteriorated by deep charge/discharge and the lifecycle is as short as five years, an extra cost is required to compensate or maintain a lead-acid battery. As the use of lead is limited and recycling of lead becomes mandatory by Restriction of the use of Hazardous Substances (RoHS) in electrical and electronic equipment (EEE), the unit price of electric power generation by a system in which a lead-acid battery is applied will increase even more.
However, in contrast to lithium, magnesium, which is used for a magnesium battery, may be obtained from salt water and minerals in a great quantity and thus magnesium is low in price, environmentally friendly, and easy to handle. In addition, the capacity per unit volume of magnesium is 3,833 mAh/cm3, which is much higher than that of lithium, 2,061 mAh/cm3, and magnesium is much more stable than lithium. Therefore, there is a great potential to develop a magnesium battery as an energy storage device for a high-capacity electric power storage system.
However, different from a lithium ion, a magnesium ion has two electric charges, showing high electric charge properties, and thus the diffusion rate of a magnesium ion becomes low as the magnesium ion is inserted into an electrode active material, for example, a positive electrode active material, due to a strong Coulomb interaction with elements constituting the positive electrode active material. As a result, the substantial capacity of a magnesium battery is reduced and the reversibility, initial efficiency, and discharge voltage of the magnesium battery are also decreased.
Therefore, there is a need for a positive electrode active material for a magnesium battery as an energy storage device for a high-capacity electric power storage system enabling reversible charge/discharge under a high voltage, an electrode and a magnesium battery including the same, and a method of preparing the electrode active material for a magnesium battery.